In many different kinds of structures, heat loss or gain can be a serious problem from the standpoint of utility cost. For example, in commercial buildings and dwellings, air conditioning and heating are major cost factors. Also in manufacturing processes in the petroleum, chemical and petrochemical industries, heat transfer can have considerable importance. For purposes of simplicity, however, heat transfer and the need for surveying it is discussed herein particularly in application to industrial processes. Such is not intended to limit the spirit and scope of this invention.
The utility cost of a petrochemical plant can be substantial. A part of the utility cost is occasioned by the necessity of maintaining the process at specified temperatures. As the cost of utilities increases, it becomes expedient to consider applying insulation to reduce heat loss. Insulation of manufacturing process equipment and other structures for heat loss reduction must provide a reasonable return on investment (ROI) to render the provision of insulation commercially feasable. It is generally too costly to simply insulate every exposed surface. Conversely, it is wasteful simply to let a substantial heat flux from the operating petrochemical plant markedly increase the cost of utilities and thus increase the ultimate cost of goods to the consumer.
In the past, heat loss surveys have been conducted mathematically, based on multiple temperature point samples from the object in question and taking into account the ambient temperature at the time of sampling, weather bureau reports of wind direction and velocity, and perhaps other factors as well. These heat loss surveys have been time consuming, expensive, and are subject to many possibilities for error.
Two modes primarily determine the rate of heat loss. That is, heat loss is occasioned by radiation of heat and/or by convection of heat. The radiation loss is particularly important when temperatures approach approximately 400.degree. F. or higher. The loss is substantial at that level, and at higher temperatures. Heat loss by radiation is thus given by the following: ##EQU1##
An additional factor is the loss of heat by convection. This refers to the loss of heat as a result of heating the surrounding air. So to speak, an outdoor situated petrochemical plant is an air cooled structure. The rate of convection loss of heat is thus given by: ##EQU2## which is added to Equation 1 to indicate heat loss by radiation and convection.
In the foregoing equations, the variables are:
E=Emissivity PA1 t.sub.s =surface temperature, degree F PA1 t.sub.a =ambient temperature PA1 v=wind velocity, ft/min PA1 Q.sub.c =convection heat loss, BTU/hr. ft.sup.2 PA1 Q.sub.r =radiation heat loss, BTU/hr. ft.sup.2
Equation 2 (in particular) is not a fundamental or exact equation; it is only one of many (perhaps equally valid) empirical equations for application in different situations--this equation relates to turbulent flow on vertical surfaces. For horizontal surfaces (roofs, for instance) or for very low wind speeds, other equations could be employed for better accuracy. It is not intended that this equation limit the present invention; its provision in this discussion is being provided solely for the purpose of example.
Q.sub.c and Q.sub.r are expressed simply as Q, indicating the rate of heat loss per unit area, BTU/hr-ft.sup.2. Surface points chosen at random for the temperature measurement are assumed to represent the average for the system, and Q is simply multiplied by the surface area of the system to calculate overall system heat loss in terms of BTU/hr.
Of the several similar theory-based methods of heat loss measurement which are common in industry at the present time, the above heat loss measurement equation appears to be the most satisfactory. This technique, however, is nevertheless ripe with opportunities for error. For example, a point much hotter or much colder than the surface average can very easily be chosen for the surface point temperature. Also, reported wind velocities seldom accurately represent the local micro-environment of surface convection. Also, adopting t.sub.a as the radiative "sink" temperature is a gross generalization, even if high-temperature equipment in the vicinity is neglected (common in industrial plants). On a clear night, due to the high absorbtivity and cold temperatures of space, the radiative "sink" would probably be much lower than the local air temperature. On a sunny day, obviously, the "sink" temperature would be somewhat higher. A more accurate technique would either avoid the need for assuming a radiative "sink" temperature or would more accurately calculate the sink temperature existing at the time of data collection.
The present invention concerns the principle of obtaining heat loss values for a (relatively) larger area based on an incremental heat loss sample (metered by any suitable means or calculated using traditional convective and/or radiative equations) and an overall temperature profile of the surface under consideration. From the sampled increment, heat losses at other portions of the surface are scaled according to the temperature profile. Increments are summed to provide total heat loss, from which a determination can be made whether the cost of insulating the surface--compared with heat loss savings--will provide an acceptable net return of investment (ROI).
Through suitable infrared photographic techniques, a thermal image of a structure can be obtained. This approach, however, will provide temperature profile lines. Even so, the heat loss is not a simple function of the surface temperature profile alone. Thus, if temperature profile lines are set at a spacing of 20.degree. F. across a surface, merely obtaining such a profile by infrared photographic techniques will not in and of itself provide data to accurately determine the overall heat loss. With the profile and scant additional (easily-collected) data, the two equations could be solved for all elements, and the results summed. In fact, this is done in situations where a fluxmeter cannot be used. Though this is a great improvement over former methods (considering the fact that minute thermal details across the whole surface are rapidly considered) without the use of a fluxmeter, the procedure still has the possibility of significant errors due to the difficulty of obtaining an accurate t.sub.a for Equation 1 and an accurate V for Equation 2 for any point on the surface. For this reason, a measured heat loss sample is obtained from one element of a surface study area whenever possible, and the boundaries of the area are chosen so that every element within the area can reasonably be assumed to have the same E, and be exposed to the same V and radiant t.sub.a.
Because there are so many variables involved, the present method simplifies and enables the taking of simple temperature measurements whereby an entire surface area (such as the surface of a petrochemical refining or processing unit) can be evaluated. This method enables heat loss of a structure to be determined typically on a per unit area basis which can be extended to the whole area or to selected portions thereof. The measurement typically provided is total BTU loss for an entire surface and is incrementally defined by BTU loss per surface area and can be obtained for any desired temperature on the surface area. Thus, data and measurements can be obtained for any area, and areas which are too cool to merit further investigation can be discarded.
While the foregoing speaks very generally of the procedure set forth in detail hereinbelow, a better understanding will be obtained upon a review of the following specification with reference to the drawings which are described below.